Variable-capacity target electrode



June 10, 1952 e. s. P. FREEMAN 2,599,754

VARIABLE-CAPACITY TARGET ELECTRODE Filed July 25, 1950 GEORGE STANLEY PERCIVAL. FREEMAN INVENTOR.

Wad/6% H/S ATTORNEY Patented June 10, 1952 UNITE STATES PATENT QFFEQE VARIABLE-CAPACITY TARGET ELECTRODE Application July 25, 1950, Serial No. 175,752 In Great Britain July 26,1949

Claims.

This invention relates to target electrodes for us in image-converting devices such as television pickup tubes or the like, and more particularly to variable-capacity target electrodes of this type.

It is well known that the light sensitivity of a storage-type television pickup tube is dependent upon the capacity between a first electrode, commonly called a signal plate or capacitive electrode, and a storage electrode on which a spacemodulated charg image corresponding to the original light image is stored before analysis by a scanning electron beam. Thus, a tube having a low capacity between the storage electrode and the signal plate has good light sensitivity at low illumination levels, but is characterized by a relatively poor signal-to-noise ratio, and, at high light levels, the signal output tends to saturate with a corresponding loss of contrast in the picture highlights. Conversely, a tube having a high capacity between the storage electrode and the signal plate requires a greater amount of illumination to achieve the minimum potential difference necessary for image storage, but is characterized by a relatively good signal-to-noise ratio in the realized video signal, and good contrast is achieved up to much higher illumination levels. It has been conventional practice to manufacture television pickup tubes with different fixed storage electrode-to-signal plate capacities to accommodate the lighting characteristics of different kinds of televised scenes, with the result that a tube ideally suited for one set of operating conditions is incapable of realizing optimum image conversion with a difierent set of lighting characteristics. Thus, in order to attain optimum performance, it has been necessary to change pickup tubes with each major change in the operating conditions.

It is a primary object of the present invention to provide a new and improved target electrode structure for use in an image-converting device in which the capacity between the signal plate and the storage electrode may be conveniently adjusted to accommodate changes in operating conditions.

In accordance with the invention, a new and improved target electrode structure for use in an image-converting device comprises a storage electrode and a capacitive electrode or signal plate,

and the two electrodes are maintained in substantially parallel adjustable space relation with respect to each other by means of a resilient deformable supporting member. Preferably, the supporting member is of the bi-metallic type, and means are provided for varying the temperature of the supporting member to control the spacing between the signal plate and the storage electrode.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals indicate like elements, and in which:

Figure l is a plan View of a target electrode structure constructed in accordance with the invention;

Figure 2 is a cross-sectional view taken along the line 2-2 of Figure 1, together with a schematic representation of an electrical circuit suitable for achieving certain objects of the invention; and.

Figure 3 is a cross-sectional view, similar to the view of Figure 2, of another embodiment of the invention.

As shown in Figures 1 and 2, a target electrode structure constructed in accordance with the invention comprises a storage electrode l0, formed of a thin insulating or semi-conducting layer H on one surface of which is supported a photo sensitive mosaic l2, and a capacitive electrode or signal plate l3. Storage electrode l0 may be peripherally secured in a mounting ring it, and a plurality of bi-metallic strip members l5 are provided for mechanically supporting capacitive electrode l3 and storage electrode ID in substantially parallel adjustable space relation with respect to each other. Each of the bi-metallic strip members l5'comprlses a pair of metal layers It and I! having diiferent temperature coefficients of expansion and welded or otherwise firmly secured to each other, and one end of each bimetallic strip is fixed to mounting ring I4 while (I/kale other end is secured to capacitive electrode As shown schematically in Figure 2, means are provided for varying the temperature of the several lei-metallic strip members l5; for example, similar high-resistance heating elements l8 may be associated with each of the bi-metallic strips and connected in series between a variable tap is associated with a potentiometer resistor 20 and ground, and a battery 2! or other suitable en ergizing source may be connected in parallel with potentiometer resistor 20.

In the absence of energizing current for heater elements l8, as when the variable tap I9 is adjusted to the bottom or grounded terminal of potentiometer 20, capacitive electrode I3 and storage electrode III are separated by a predetermined mutual spacing which affords a fixed capacity for the target electrode structure. This fixed capacity, may, for example, correspond to a minimum capacity in order to obtain high light sensitivity for low illumination levels. If tap I9 is moved continuously toward the free or highpotential terminal of potentiometer 20, the temperature of bi-metallic strip supporting members I is increased, and, if the outer layer II of each strip member has a higher temperature coefficient of expansion than the inner layer I6, the distance between capacitive electrode I3 and storage electrode I0 is progressively decreased with a consequent increase in capacity. To achieve this type of operation, the outer layers Il may be of an alloy of 22% nickel, 3% chromium, and the balance iron, or of an alloy of 72% manganese, 18% copper, and nickel, while the inner layer :6 may be of an alloy of 36 nickel and 64% iron. These alloys are specified merely by way of example, as other bi-metallic alloys suitable for use in high-vacuum are known to the art.

On the other hand, the target electrode may be constructed in such a manner that the capacitive electrode I3 is originally spaced from storage electrode Iii by a minimum distance corresponding to high capacity for use in televising scenes where illumination is ample and a good signalto-noise ratio is desired. In this event, the outer layer I! of each bi-metallic strip member is constructed of a material having a lower temperature coefilcient of expansion of that of the inner layer I6, so that an increase in the temperature of the supporting members, under control of variable tap I9, results in an increase of the spacing between the electrodes and a consequent decrease in capacity. Merely by way of illustration, and in no sense by way of limitation, a wide range of operating conditions may be accommodated if the target electrode is so constructed and arranged that the spacing between capacitive electrode I3 and storage electrode I0 may be altered from a minimum of 0.0005-inch to a maximum of 0.1- inch.

In the embodiment of Figures 1 and 2, the storage electrode ID comprises a thin semi-conducting layer I I, and a photosensitive mosaic is supported on the surface of semi-conducting layer I I facing capacitive electrode I3. A target electrode of this type is particularly adaptable to an imageconverting device of the orthicon type, in which the surface of the storage electrode remote from capacitive electrode I3 is scanned by a lowvelocity electron beam. However, the invention is not limited to an electrode arrangement of this type. For example, as shown in Figure 3, the storage electrode may comprise a thin layer 22 of insulating material, and a photosensitive mosaic 23 may be deposited on the surface of insulating layer 22 remote from capacitive electrode I3. A target electrode structure of this latter type may be particularly useful in an image-converting device of the iconoscope type using a highvelocity scanning beam.

While four bi-metallic strip members have been illustrated for supporting the capacitive electrode and the storage electrode in mutual space relation, it is apparent that the number of supporting members may be chosen at will. However, it is preferred that at least three supporting members be used in order to insure that the two electrodes are always maintained in parallel relation with respect to each other. Moreover, while high-resistance heater elements have been shown for controlling the temperature of the bi-metallic strips, it is also within the scope of the invention to connect the electrical circuit through the strip members themselves to provide the desired temperature control.

The invention may also find application in the construction of target electrodes for certain types of image-storage tube, in which storage of a charge image is effected by a suitably modulated writing beam and subsequent utilization of the stored charge image is eifected by scanning the storage electrode with a reading beam. In this application, the change in capacity which may be efiected by controlling the spacing between the electrodes serves to control the removal rate of the stored signal, and also to control the total amount of stored charge.

In some embodiments of the invention, it may be desirable to construct capacitive electrode I3 as a light-transparent layer of conductive material or as a fine conductive mesh to permit the passage of light for energizing a photosensitive mosaic. In other embodiments, capacitive electrode I3 may be constructed as a metal foil or as a thin electron-permeable metal membrane to permit penetration by a scanning beam which functions either to store information on the storage electrode or to remove stored information therefrom. Furthermore, storage electrode I0 may be of either the single-sided or doublesided variety, and the mosaic may be constructed of either a photosensitive material or a secondary-electron-emissive material, depending upon the application for which the target is intended. In all embodiments, the target electrode structure is adapted to provide a convenient control over the inter-electrode spacing, in order to adapt the tube to optimum operation over a wide range of operating conditions. V

Thus, the present invention provides a new and improved target electrode structure for use in an image-converting device. By virtue of the invention, a single television pickup tube may supplant two or more tubes of different characteristics in conventional applications. A convenient control over the spacing between the electrodes constituting the target structure is provided in order to permit ready changes in the light inputto-signal output characteristic of the tube to meet changes in operating conditions.

While particular embodiments of the present invention have been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A variable-capacity target electrode structure for an image-converting device comprising: a storage electrode; a capacitive electrode; and a resilient deformable member mechanically supporting said electrodes in substantially parallel adjustable space relation with respect to each other.

2. A variable-capacity target electrode structure for an image-converting device comprising: a storage electrode; a capacitive electrode; and a bi-metallic member mechanically supporting said electrodes in substantially parallel adjustable space relation with respect to each other.

3. A variable-capacity target electrode structure for an image-converting device comprising: a storage electrode; a capacitive electrode; and at least three oi-metallic strip members mechanically supporting said electrodes in substantially parallel adjustable space relation with respect to each other.

4. A variable-capacity target electrode structure for an image-converting device comprising: a storage electrode; a capacitive electrode; a bimetallic member mechanically supporting said electrodes in substantially parallel adjustable space relation with respect to each other; and means for varying the temperature of said bimetallic supporting member to control the spacing between said electrodes.

5. A variable-capacity target electrode structure for an image-converting device comprising: a storage electrode; a capacitive electrode; at

GEORGE STANLEY PERCIVAL FREEMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,231,960 Smith Feb. 18, 1941 2,435,316 Larson Feb. 3, 1948 2,437,418 Cawein Mar. 9, 1948 

